The present invention relates to the art of packaging semiconductor devices, and more particularly, to an improved silver-glass paste which is deposited in a viscous state between a die and a ceramic substrate and then fired to form a secure bond therebetween.
Silver-glass pastes have been used for a number of years to attach silicon die to ceramic substrates. U.S. Pat. Nos. 3,497,774 of Hornberger et al.; 4,101,767 of Dietz et al.; and 4,761,224 of Husson et al. disclose examples of such pastes. Typically they include a high lead glass frit, silver particles and a suitable organic vehicle. This vehicle is typically made of a resin and a solvent. During an initial drying phase the liquid vehicle evaporates. Upon firing, the remaining organic vehicle, i.e. the resin, decomposes, the glass softens to wet the ceramic and the silver flake sinters together.
The principal reason for using a resin in the organic vehicle of a die attach paste is for solvent spread attenuation. If only a solvent were used it would quickly spread across the surface of the ceramic substrate carrying glass and silver particles with it. The resin provides a suitable entrainment mechanism that retards such solvent spread.
The traditional vehicle system used in silver-glass die attach pastes consists of one or more high boiling organic solvents and a resin. The combined organics are required to volatilize and/or decompose cleanly during the curing of the product to leave the silver-glass composite free of carbonized residues. The clean organic burnout requirement has essentially restricted the polymers used as resins in this system to the following categories: (1) polyalkyl methacrylates; (2) nitrocellulose (with nitrogen content.gtoreq.12.0%); (3) polyisobutylenes; and (4) polypropylene gylcols. Polymer's of alpha-methylstyrene represent a fifth potentially useful class of resins that upon thermolysis yield completely volatile decomposition products. Their use, however, would be restricted to cure under non-oxidizing conditions.
Resins have been shown to impart the following beneficial properties in order of decreasing importance: (1) reduction in solvent bleed from the perimeter of the uncured paste fillet; (2) reduction in the rate of settling of the paste solids; (3) facilitation of the dispersion and wetting of the glass and silver; and (4) provision of green strength to paste in the dried state.
Practical considerations have limited the options for resin selection. The polyalkyl methacrylates have been shown to possess the best blend of properties for use in die attach application. The preferred methacrylate polymers include those generated from esters of the lower alcohols (i.e. where the alkyl is methyl, ethyl, propyl or butyl).
Use of the commercially available polyalkyl methacrylates has proven to be problematic. It has been demonstrated that whenever sufficient methacrylate resin was used to curb solvent spread that the resulting pastes had excessive tailing when dispensed from automatic die bond equipment. Tailing or stringiness has a major negative impact on the application of these pastes, especially when they are dispensed from equipment configured with a starfish nozzle. The downsides from this problem included a reduction of product throughput and/or an increased yield loss neither of which could be tolerated by paste users.
It is well known that the molecular weight of the polymer used has a pronounced effect on solvent spread. Generally the higher the molecular weight the lower the degree of solvent spread for any given weight percent of resin dissolved in the vehicle solvent. Presumably, the dispensability of the paste could be improved by incorporation of the minimum amount of high molecular weight resin capable of solvent spread attenuation. Unfortunately, this approach is thwarted by the simultaneous increase in stringiness of the polyalkyl methacrylates as molecular weight is increased. A paste formulation based on the commercial methacrylate resins must of necessity represent a compromise between low solvent spread and acceptable dispensing properties which cannot be simultaneously optimized.